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Re: GEOSTATS: FYI -Reply

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  • charliekufs@juno.com
    Jennifer, You have your answer in front of you. The average yields ARE noticeably different. You can see the differences in graphs and other methods. Whether
    Message 1 of 2 , Jul 5, 1999
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      Jennifer,
      You have your answer in front of you. The average yields ARE noticeably
      different. You can see the differences in graphs and other methods.
      Whether or not the 3 to 20 yield differences are meaningful is something
      you'll have to decide. However the variances are large, even when based
      on 3000 samples. This is very common in environmental data. It means
      either that the natural variability is inherently large or that you have
      not controlled enough extraneous variance to detect a statistically
      significant difference.

      So what do you do? Here are some options.

      1. Look into an analysis of variance (or covariance, depending on your
      variables) design that will control additional variance. If you have data
      that can be used in an ANCOVA design, you might be able to detect the
      differences.

      2. Just report what you found. Show the graphs and other analyses. But
      instead of the statistical tests, report confidence limits. They convey
      the same information without the stigma of not being significant.

      Charlie Kufs

      ~~~~~ TERRABYTE CORPORATION
      ~~~~~ Statistics and Computer Technologies
      ~~~~~ Applied to the Environmental Sciences
      ~~~~~ http://members.home.net/terrabyte
      ~~~~~ terrabyte@...

      On Mon, 5 Jul 1999 09:22:44 -0600 "Gary Sallows"
      <SallowsG@...> writes:
      >Following is a message from one of my graduate students. Any
      >suggestions
      >for alternative methods to compare management zones for
      >significant
      >differences would be appreciated.
      >
      >Steve Searcy
      >----------------------------------------------
      >Hi,
      >
      >I'm a graduate student at Texas A&M working in the precision farm
      >area through the ag engineering department. My advisor, Steve
      >Searcy, suggested that write to this group and ask for suggestions.
      >
      >I have broken a 100 acre field into 20 management units based on
      >some criteria. I have very intense yield data. On average, there
      >are 3000 yield points within one management unit. I'm trying to
      >prove that the management units that are adjacent are different
      >from each other (based on yield). The statistical tests that I run
      >tell
      >me that my results are not significant (the adjacent management
      >units are
      >not different); however, when I graph the results I can see
      >differences in
      >average yields. I also tried a variety of other methods to determine
      >management units, and I'm getting the same results.
      >
      >I am running a Student's t-test. The difference in the yield
      >average between the adjacent management zones
      >ranges from 3 to 20 and I have very high variances. Within the
      >approximately 3000 points that I have in one management unit, the
      >yield
      >ranges from 8 to 238 which accounts for the high variance (300-
      >400). Due
      >to the high variance and small difference between the yield
      >averages, the
      >t value is very small (around .1 to .01). When I compare this value
      >to a
      >Bonferroni t value (95% confidence) it is very insignificant (the
      >Bonferroni t value is around 2.4).
      >
      >I get the impression that these types of statistical tests have been a
      >problem with analyzing precision farming techniques. I was
      >wondering if
      >anyone had any suggestions for me concerning other statistical
      >tests or
      >other ways to prove differences with this kind of data.
      >
      >Sincerely,
      >Jennifer Jessip
      >AGEN-Research Assistant
      >
      >
      >*************************************************************************
      >Stephen W. Searcy E-mail: s-searcy@...
      >104
      >AERL Phone: 409-845-3668 West
      >Campus
      >Rt. 4A Fax: 409-8478627 Texas A&M University
      >College Station, TX 77843
      >
      >
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